Stress–Strain Behavior of Nicalon-Fiber-Reinforced Calcium Aluminosilicate Composites under Tensile Fatigue Conditions

The unusual stress–strain hysteresis loop shape exhibited by ceramic-matrix composites under cyclic loading has previously been explained as a result of either strain rate dependence of the frictional shear stress or crack closure. This investigation has determined that the response is due to neither mechanism. Instead, it is proposed that a variation of interfacial shear strength occurs during each cycle. A static coefficient of friction dominates immediately after loading or unloading. A much lower dynamic coefficient of friction operates once fiber sliding begins. This dynamic coefficient appears to be very dependent on surface roughness.     

Influence of Interfacial Sliding Stress on Fatigue Behavior of Oxidized Nicalon/Calcium Aluminosilicate Composites

The influence of the interfacial sliding stress on the steady-state tensile fatigue behavior of a Nicalon/calcium aluminosilicate composite has been characterized experimentally. Characterization occurred in an experiment where the microstructure and properties of the carbon interface were altered by oxidation at increasingly greater temperatures. After each oxidation step, the steady-state energy dissipation was quantified. Using a model, changes in the interfacial sliding stress were determined after each oxidation step. The results revealed a minimum in the sliding stress after oxidation in the temperature range of 100°-300°C that corresponded to a maximum in the amount of energy dissipated by frictional sliding.     

Effect of Quasi-Static Loading Rate on Damage Formation in Silicon Carbide Reinforced Calcium Aluminosilicate Unidirectional Composites

For experiments at higher and lower loading rates, significant differences in the mechanical responses of unidirectional silicon carbide reinforced calcium aluminosilicate composites were observed. Axial and transverse stress–strain measurements, acoustic emission measurements, and post-test microstructural observations all indicated that there are changes in the sequence and extent of the formation of damage. The differences are attributed to rate-dependent matrix cracking due to environmental effects and rate dependencies associated with the fiber–matrix interface.     

Static and Cyclic Fatigue of Alumina at High Temperatures

Fatigue behavior of alumina at 1200°C was investigated. Uniaxial tensile tests were conducted in both static and cyclic loading. A variety of loading wave forms were applied during the cyclic tests. Cyclic lifetime is found to be cycle shape dependent and controlled by the duration of the hold time at the maximum tensile stress in a cycle. Cyclic loading with a higher strain rate and a short duration of maximum stress during each cycle provides a beneficial effect on lifetime in comparison to static loading at the same maximum stress. The time to failure for cyclic loading with a longer hold time at maximum stress is very close to the static loading lifetime. Viscous boundary phase may be the primary contributor to the improved cyclic fatigue resistance for cyclic loading with a short duration of maximum stress.     

Static and Cyclic Fatigue Behavior of a Polycrystalline Alumina

The fatigue behavior of a polycrystalline alumina was investigated. Stress conditions consisted of a static tensile stress and a static tensile stress with superposed sinusoidal cyclic stress. The alumina exhibited the expected static fatigue behavior; a cyclic fatigue effect characterized by a frequency and amplitude dependence was also observed. Possible mechanisms of cyclic fatigue in brittle materials are discussed.     

Cyclic Fatigue Life‐ and Crack‐Growth Behavior of Zirconia–Niobium Composites

A 3Y‐TZP/Nb composite fabricated by Hot‐pressing (HP) sintering with well‐distributed lamellar/flake–shaped metal particles (20 vol% fraction) has been studied under cyclic loading. Fatigue life was determined for the ceramic/metal composite as well as for monolithic zirconia to compare the sensitivities of both materials to cyclic stresses. In both cases, the fatigue test was performed according to ISO 6872. It was found that the 3Y‐TZP/Nb composite exhibits fatigue behavior which was compared with monolithic zirconia. The growth of fatigue cracks influences the bridging actions of the metallic grains and causes significant degradation in the mechanical properties of the composite material.     

Subcritical Crack Propagation in 3Y-TZP Ceramics: Static and Cyclic Fatigue

A detailed analysis of crack propagation in tetragonal zirconia polycrystals doped with 3 mol% of Y₂O₃ (3Y-TZP) ceramics is presented. Crack propagation tests have been conducted for crack velocities of 10^-12-10^-3 m/s in several environments, including air, water (in the temperature range of 3°-85°C), secondary vacuum (10^-5 mbar), and silicon oil. Analysis of the experimental results-three propagating regimes that are dependent on the environment and a marked threshold below which no propagation occurs-shows that stress corrosion by water molecules is the key mechanism for crack propagation. The effect of grain size on the crack velocity is quantified and analyzed in terms of transformation toughening. Experiments under cyclic loading have been conducted to quantify the effects of cyclic fatigue. Crack velocities are higher under cyclic loading than that predicted by stress corrosion alone, and the threshold is lower. Experiments that have been conducted at two different frequencies (0.1 and 1 Hz) and static-fatigue/cyclic-fatigue sequences show that both stress corrosion by water and pure cyclic-fatigue effects are operative under alternative stresses.     

Mechanical and Environmental Factors in the Cyclic and Static Fatigue of Silicon Nitride

The effects of environment on cyclic and static fatigue behavior were investigated with hot-pressed silicon nitride materials. Tests were conducted at ambient temperature on standard compact tension specimens, and a dc electric potential technique was used to monitor crack lengths in situ. The results indicate that the environmental sensitivity of our materials under both cyclic and static loading mirrors that of durable glasses in static fatigue. The materials were most sensitive to water in the environment, while changes in pH had no significant effect in the range tested. In addition, NH₃ was much less reactive with our materials than with vitreous SiO₂. In some cases, the intergranular glass appears to be the site of environmental interaction. Evidence was also found that cyclic fatigue is not simply a manifestation of static fatigue. Cyclic fatigue was seen to occur in the absence of measurable static fatigue, and the data indicate that the mechanism of cyclic fatigue involves damage to the crack wake shielding zone.     

Fatigue Damage Accumulation in 3-Dimensional SiC/SiC Composites

The effect of fatigue loading on the mechanical performance of 3-D SiC/SiC composites was investigated. A non-destructive macromechanical approach was applied which permits for the evaluation of the material damage state by monitoring its dynamic response as function of fatigue cycles. The correlation of the results provided by this method to that of other non-destructive techniques such as Acoustic Emission (AE), leads to a detail micromechanical-macromechanical monitoring of the material fatigue behaviour. The damage modes identification and their successive appearance, together with the evaluation of the material performance at the different stages of fatigue loading, is among the inspection capabilities that provides the above mentioned combination of non-destructive techniques. The proposed methodology applied in the case of a 3-D SiC/SiC ceramic matrix composite material and the effect of fatigue loading on the material integrity was evaluated by measuring the degradation of the dynamic modulus of elasticity and the increase of the material damping. Conclusions, concerning design aspects using these materials, as well as fatigue life prediction were provided. Finally, the sensitivity of the proposed methodology for the definition, the characterisation of the development and the separation of the different damage modes during fatigue loading has been discussed.     

Cyclic Fatigue Crack Propagation in Alumina under Direct Tension—Compression Loading

Cyclically induced crack propagation occurs in alumina subjected to direct tension—compression loading. The crack increment per cycle (da/dN) has a power-law dependence on the peak stress intensity factor (K_max). Cyclic crack growth can occur at lower values of K_max than are required to produce static fatigue effects. Subcritical crack-growth behavior was found to be dependent on specimen geometry: it is suggested that direct compressive loads and crack length are both factors that affect cyclic fatigue behavior, and that the use of K alone to characterize fatigue crack growth in ceramics may be questionable.     

High-Temperature Failure of an Alumina-Silicon Carbide Composite under Cyclic loads: Mechanisms of Fatigue Crack-Tip Damage

Experimental results are presented on the mechanisms of tensile cyclic fatigue crack growth in an A1₂0₃-33-vol%-SiC-whisker composite at 1400°C. The ceramic composite exhibits subcritical fatigue crack propagation at stress-intensity-fator values far below the fracture toughness. The fatigue characterized by the stressintensity-factor range, ΔK, and crack propagation rates are found to be strongly sensitive to the mean stress (load ratio) and the frequency of the fatigue cycle. Detailed transmission electron microscopy of the fatigue crack-tip region, in conjunction with optical microscopy, reveals that the principal mechanism of permanent damage ahead of the advancing crack is the nucleation and growth of interfacial flaws. The oxidation of Sic whiskers in the crack-tip region leads to the formation of a silica-glass phase in the 1400°C air environment. The viscous flow of glass causes debonding of the whisker-matrix interface; the nucleation, growth, and coalescence of interfacial cavities aids in developing a diffuse microcrack zone at the fatigue crack tip. The shielding effect and periodic crack branching promoted by the microcracks result in an apparently benefcial fatigue crack-growth resistance in the A1₂0₃—SiC composite, as compared with the unreinforced alumina with a comparable grain size. A comparison of static and cyclic load crack velocities is provided to gain insight into the mechanisms of elevated temperature fatigue in ceramic composites.     

Tensile Creep and Creep-Strain Recovery Behavior of Silicon Carbide Fiber/Calcium Aluminosilicate Matrix Ceramic Composites

The tensile creep and creep strain recovery behavior of 0° and 0°/90° Nicalon-fiber/calcium aluminosilicate matrix composites was investigated at 1200°C in high-purity argon. For the 0° composite, the 100-h creep rate ranged from approximately 4.6 × 10⁻⁹ s⁻¹ at 60 MPa to 2.2 × 10⁻⁸ s⁻¹ at 200 MPa. At 60 MPa, the creep rate of the 0°/90° composite was approximately the same as that found for the 0° composite, even though the 0°/90° composite had only one-half the number of fibers in the loading direction. Upon unloading, the composites exhibited viscous strain recovery. For a loading history involving 100 h of creep at 60 MPa, followed by 100 h of recovery at 2 MPa, approximately 27% of the prior creep strain was recovered for the 0° composite and 49% for the 0°/90° composite. At low stresses (60 and 120 MPa), cavities formed in the matrix, but there was no significant fiber or matrix damage. For moderate stresses (200 MPa), periodic fiber rupture occurred. At high stresses (250 MPa), matrix fracture and rupture of the highly stressed bridging fibers limited the creep life to under 70 min.     

Static and Cyclic Fatigue Behavior of a Sintered Silicon Nitride at Room Temperature

The static and cyclic fatigue behavior of sintered silicon nitride was investigated at room temperature. Flexure specimens, with an indentation-induced flaw at the center, were tested under a static or cyclic load applied by four-point bending. Sintered silicon nitride was shown to be susceptible to static and cyclic fatigue failure. Comparing the static and cyclic fatigue lifetimes at frequencies from 0.01 to 10 H z , it was shown that minimum time to failure was almost the same, in spite of differences in loading mode or frequency. However, cyclic stress decreased the scatter in lifetime by reducing the upper limit. Moreover, the cyclic fatigue limit was significantly lower than the static fatigue limit. High-magnification fractography revealed a fatigue failure dominated by intergranular cracking with partial transgranular failure at perpendicularly elongated crystals. This suggests that the intergranular fatigue crack can be arrested at grain-boundary triplets, and also can be reactivated by subsequent cyclic loading. The crack growth rate, calculated from the fatigue lifetime, showed three characteristic regions having a plateau at 70% to 90% of the fracture toughness, which suggests a possible intergranular stress corrosion cracking mechanism resembling that in glass or alumina.     

Ion‐Exchanged Lithium Aluminosilicate Glass: Strength and Dynamic Fatigue

Sodium for lithium and potassium for lithium ion‐exchanges of a lithium aluminosilicate glass were conducted and the resulting strength and dynamic fatigue characteristics were studied. Four‐point bend mechanical tests revealed that greater strengthening can be achieved by the potassium for lithium ion‐exchange, compared to the sodium for lithium ion‐exchange, and that the dynamic fatigue tendency is strongly suppressed by both exchanges. This suppression of dynamic fatigue characteristics of ion‐exchange strengthened glass was explained by the ability of the surface compressive layer to delay the onset of slow crack growth. Bulk stresses continue to increase in magnitude while the crack is arrested in the surface compressive stress region. Upon offsetting the surface compressive stress, the crack rapidly propagates into a high‐magnitude tensile stress field until the fracture toughness is reached, resulting in minimal crack growth prior to material failure. A slow crack growth model utilizing a fracture mechanics weight function was developed to simulate the experiments. Dynamic fatigue characteristics of the as‐received glass, without ion‐exchange treatment, were also measured and simulated for comparison.     

Fatigue Damage in Ceramic Coatings From Cyclic Contact Loading With a Tangential Component

The role of a tangential load component in cyclic contact-induced failure of a brittle coating layer is investigated. Tests are conducted on model bilayer systems consisting of glass plates bonded to polymeric substrates, using a spherical indenter in periodic off-axis loading, in a water environment. The principal damage is that of a partial cone crack which forms in the wake of the contact and propagates steeply through the coating layer with continued loading. The critical number of cycles required to propagate the cone cracks through the coating is substantially reduced in off-axis relative to axial loading, and diminishes rapidly with increasing peak load. It is confirmed that the superposition of sliding tractions at the contact can have a profoundly deleterious effect on coating lifetime.     

Static and Cyclic Fatigue of Alumina at High Temperatures: II, Failure Analysis

Failure mechanisms of an alumina, tested at 1200°C under static and various cyclic loading conditions, were examined. Slow crack growth of a single crack is the dominant mechanism for the failure in specimens under cyclic loading with a short duration of maximum stress at all applied stress levels, as well as at high applied loads for static loading and cyclic loading with a longer hold time at maximum stress. At low stress levels, failure of static loading and cyclic loading with a longer hold time at maximum stress might occur by formation and/or growth of multiple macrocracks. More importantly, for all the given loading conditions. The viscous glassy phase behind the crack tip could have a bridging effect on the crack surfaces. A simplified model for calculating effective stress intensity factor at the crack tip under static and various cyclic loading demonstrated a trend consistent with the stress–life data.     

Damage and Failure in Unidirectional Ceramic–Matrix Composites

A study of the mechanical characteristics of a unidirectional fiber–reinforced calcium aluminosilicate matrix composite has been conducted. The properties have been related to the individual properties of the matrix, the fibers, and the interfaces, as well as the residual stress, using available models of matrix cracking and fiber fracture. Comparisons have been made with lithium aluminosilicate matrix composites. Predictions of initial matrix cracking and of ultimate strength using the models are found to correlate well with the measured values. However, deficiencies have been noted in the ability of the models to predict the evolution of matrix cracks, plus associated changes in the modulus.     

Crack Propagation Behavior of Polycrystalline Alumina under Static and Cyclic Load

Various causes for cyclic-loading fatigue in ceramics have been proposed. Degradation in the grain-bridging effect is the most important cause for cyclic-loading fatigue in nontransforming ceramics. Cyclic- and static-loading crack propagation behavior in terms of crack propagation rate, load—strain curve, and R -curve in compact tension specimens of polycrysalline aluminas with two types of average grain size is reported. Significant bridging is observed in coarse-grained alumina. The results are consistent with the proposal that grain bridging increases with grain size and that degradation in grain bridging is the most important cause for cyclic-loading fatigue in alumina ceramics.     

Microstructural Study of the Hot Corrosion of a Calcium Aluminosilicate Glass-Ceramic and a Si-C-O-Fiber-Reinforced Calcium Aluminosilicate Matrix Composite via Sodium Sulfate in Air and Argon at 900°C

The hot corrosion of a calcium aluminosilicate (CAS) glass-ceramic and a composite of CAS matrix that has been reinforced with Si-C-O (Nicalon) fiber has been investigated by X-ray diffractometry, scanning electron microscopy, and transmission electron microscopy. Samples of the monolithic CAS and the Si-C-O-CAS composite were subjected to corrosion using liquid sodium sulfate at 900°C for 50 h in air and argon environments. The monolithic and composite samples both were corroded by sodium sulfate, and corrosion damage in the composites was more severe than in the monolithic CAS, irrespective of the gaseous environment. The increased corrosion damage in the composites was due to the presence of Si-C-O fibers, which changes the mechanism of corrosion. The corrosion products in monolithic CAS were different from those in the composites; this disparity was also due to the presence of the fibers in the composite. The corrosion zones in all the samples were severely cracked, and the cracks extended into the unaffected regions of the samples. Mechanisms of hot corrosion have been proposed and discussed for both the monolithic and composite samples.